1. Field of the Invention
The invention pertains to apparatus and methods for making carbon fiber from polymer precursors and more particularly to apparatus and methods for using microwave assisted plasma processing to carbonize or graphitize fibers on a continuous or semi-continuous basis.
2. Description of Related Art
The most significant obstacle to the widespread use of high-strength, light-weight, carbon fiber based composites by the consumer industry is the high cost of carbon fibers in comparison to lower-strength but heavier conventional structural materials such as steel. This has limited the application of carbon fibers to reinforcing materials in advanced polymer matrix composites. Currently, around 45,000 tons of carbon fibers are produced annually throughout the world. Although this number may seem substantial, the commercial industry has yet to realize the potential widespread use of carbon fibers because of the high costs associated with their production as compared to other materials. The most significant cost factors include the high cost of carbon precursors (45-50% of production costs), the high cost of equipment and energy consumption (20-25% of production costs), and the cost associated with a limited production rate for a quality product. The carbonization, graphitization, and surface treatment stages represent approximately 13%, 15%, and 2%, respectively, of the overall conventional manufacturing cost.
Carbon and graphite fibers are conventionally produced through the controlled pyrolysis of fibrous organic carbon precursors such as PAN, mesophase pitch (petroleum or coal tar), rayon or other polymeric based precursor in what is generally a lengthy and expensive process. The precursor is spun into fibers, stabilized by oxidation at a relatively low temperature (200 to 300° C.), then carbonized at a very high temperature (1300 to 2500° C.), under tension in a conventional graphite brick-lined furnace. The overall carbon fiber manufacturing process requires several hours to produce high-quality carbon fibers, as shown schematically in FIG. 1. The PAN precursor 1 is fed from spool 2 to pretreatment stage 3, to an air oxidation furnace 4 at atmospheric pressure. The fully oxidized PAN precursor 5 enters the carbonization and graphitization furnaces 6 and 7, respectively, which are pressurized to one atmosphere of nitrogen to preclude any further carbon fiber oxidation. The fully graphitized carbon fiber then enters a surface treatment stage 8 and emerges as a fully graphitized and surface treated carbon fiber 9. Sizing and subsequent drying are applied in stages 10 and 11, respectively. The final carbon fiber product is then wound on spool 12 for delivery to the customer.
Three process stages of carbonization 6, graphitization 7 and surface treatment 8 represent much of the time and cost in the overall carbon fiber manufacturing process. The high temperatures in the carbon fiber manufacturing process cause the initial organic material in the fiber to convert into carbon while the fiber's noncarbon elements are expelled in the form of volatile gases.
Carbon fibers are usually produced in bundles (tows) having 1,000 to 100,000 individual filaments. The average fully- or partially-oxidized carbon fiber precursor filament starts out with a diameter of about 10 μm and shrinks to about 7 μm at the end of carbonization process as unwanted components (effluent) in the filaments containing elements such as oxygen, nitrogen, and hydrogen slowly diffuse to the surface. The carbonization process is time consuming and expensive because of power requirements and furnace maintenance costs. Attempts to speed the carbonization process can produce volatile gases such as carbon monoxide and hydrogen, which can create bubbles or cracks in the filaments depending upon quality of precursor and stabilization recipes. These bubbles are structural flaws that substantially weaken the fibers. Therefore, a need exists for a carbon fibers manufactured at lower cost with acceptable automotive-quality mechanical properties.
In the case of PAN, approximately half of the weight of the tow is lost during the carbonization stage. This weight loss consists of converted gases or effluents. Some of these gaseous components are hazardous, so these gases are usually passed through a conventional scrubbed incinerator. Considerable quantities of greenhouse gases are produced in the incineration process. The required incineration of the effluents of carbon fiber production adds manufacturing costs that must be passed along to the customer. Incinerators must operate within their required permitting range and this represents a regulatory limitation on the quantity of carbon fiber production at a given plant.
Early work at Oak Ridge National Laboratory on carbon fiber manufacturing is covered by a number of patents. Use of plasma processing technology for the production of carbon fiber is disclosed in U.S. Pat. No. 6,372,192. This patent discloses simple batch processing of small samples of fully or partially oxidized PAN precursor wound on an quartz frame to support and tension the precursor during plasma processing. The processing tension on the fiber is generated through the natural fiber shrinkage on the fixture. The study proved that batch plasma processing of carbon fiber is feasible on a laboratory scale. Continued work in this area by Paulauskas et al. was disclosed in U.S. Pat. No. 6,375,875, which describes the development of a diagnostic monitor for carbon fiber processing that measures the dielectric properties of carbon fiber using a non-contacting, resonant microwave cavity. U.S. Pat. No. 6,514,449 describes a process to modify the surface topography of graphite fiber via the combined action of microwave and plasma energy. Well-controlled plasma processing was shown to be able to modify the microscopic features on the surface of graphite fiber, which would enhance the interlocking of the resin matrix to the fibers in a composite.
U.S. Pat. No. 3,607,062 describes an applicator used only to graphitize carbon fiber at radiofrequencies less than 500 MHz. The device requires very high electric fields established in a small gap between two electrodes to heat the carbonized fibers from 2000-3500° C. The electric fields along the carbon fiber axis heat the carbon fiber via the principle of dielectric heating using the radiofrequency energy. The high electric fields were produced using a quarter wavelength reentrant coaxial cavity that was tuned to resonance. The electric fields needed to be high because the dielectric heating scales as the product of the frequency times the imaginary (lossy) part of the dielectric constant times electric field squared. Because the heating frequency is very low compared to 2450 MHz, and because the dielectric losses of the carbon fiber are small at lower frequencies, the electric field needed to be very high to make up for the smaller terms in the dielectric heating equation. Unfortunately, a small gap operating at very high electric fields is very prone to dielectric breakdown in the air gap between the electrodes. This would not be very reliable in a production environment. Another problem with the system of '062 is that carbon fiber quickly becomes highly electrically conductive as it is processed from fully carbonized to fully graphitized. As the carbon fiber begins to heat at the beginning of the gap, it starts to transition from a lossy dielectric to a good electrical conductor. However, the electric fields in the small gap can be easily shorted out because the filament begins to conduct as it is heated and this would limit dielectric heating and would tend to cause high voltage breakdown in the gap. Processing the filament with such a system would be very delicate because the carbon fiber filament needs to exit the high field gap before it can cause breakdown, but if the filament residence time is too short, the filament never is fully processed. The heating process would tend to be self-limiting and not very reliable because of electric field breakdown. In addition, '062 is oriented toward single filaments whereas industrial practice requires a system able to process complete tows consisting of thousands of filaments. The processing length is a very small gap in order to generate the high electric fields needed; whereas a processing length of several meters would be more desirable so the residence time could be much higher for a given line speed and power input.
U.S. Pat. No. 5,037,464 describes a method for manufacturing carbon-coated optical fibers using a microwave-generated plasma. The protective carbon-coating layer is formed via carbon vapor deposition on the optic fibers. This is not a method for manufacturing carbon fibers but merely a way to coat or surface clean an existing optical, not carbon, fiber.
U.S. Pat. No. 4,197,282 describes a method to sequentially batch manufacture only carbon fibers using pitch-based precursors exclusively. The pitch was obtained from residues of the distillation of coal and petroleum. The process is inherently a systematic batch process not suitable for continuous processing applications. No plasma processing is employed in the manufacturing process. Prior to the application of microwave for graphitization, the precursor requires extensive pretreatments, both thermal and chemical, to achieve conditions that are essential for the coupling of the microwave energy to the fiber. This pretreatment is required in order to obtain a high content of carbon in the precursor and because the chemistry between PAN and pitch-based precursors are completely different.
U.S. Pat. No. 6,749,827 describes a method for growing carbon fiber from single-walled nanotubes. The single-wall nanotube is required to act as a seed or substrate for the subsequent formation of the carbon fiber. The carbon comes from a gaseous carbon source produced by a high power laser. The process is primarily a laboratory technique and was not specifically adapted to the continuous manufacture of carbon fiber.
U.S. Pat. No. 3,824,398 describes a method for generating a radio frequency coaxial discharge plasma between an inner electrode on axis and an outer electrode that could be used to treat carbon fiber tows. The patent also describes a method to isolate the pressure in the plasma from the external atmosphere by using an air lock consisting of a pair of small ducts roughly the same diameter as the tow, and spaced close together along the duct that encloses the tow.
US Patent Application No. 2003/0026980 discloses a method to produce hollow carbon tubes and fibers. These fibers are produced in the nano-scale range of sizes. In general, this method is based on a multi-batch process and the carbon fibers and tubes are grown on a micro-seed as a substrate. This method is not suitable for producing carbon fibers on an industrial scale.
US Patent Application No. 2003/0051993 discloses the plasma activation of some types of chemical reactions. It is a plasma-assisted chemical processing method. The applications uses a non-thermal capillary discharge plasma that is, in general, not very uniform over a large processing area. This method limits itself to the application of plasma with chemical feedstocks, such as hydrocarbons, to chemically oxidize or to produce functional chemical groups on the processed material.